Here are my first thoughts on a set of loose asset guidelines for all projects using Tamriel-Data. These are not meant as absolute rules but as points of orientation for everyone participating in the creation of models. For that reason, I'd like to hear our modellers and textures on this, in particular. Other than that, everyone is invited to comment and complement.

Tamriel Data - Asset Creation Recommendations

Textures:

Color:
Use a similar lightness and saturation than comparable objects that already exist in the game.
The model with your texture should not look completely offstyle, when placed next to other things in the game environment. There are several levels on which this can be ensure, but using a similar lightness, saturation and contrast is a good first step and a possible least common denominator for the different projects.

Material consistency:
Have an eye on the inner logic of the universe. While the absense of material consistency is not necessarily noticed by the player and not necessarily a fault in itself, it's presence is likely going to be noticed and serve to increase immersion.
For example:
Use leathers or furs that can actually be derived from animals in the environment (like netch leather).
Use a certain color tint for iron, steel, silver, ebony, etc. matching its look everywhere else in the game.
Use a similar canvas backdrop for all textures consisting of a certain type of fabric.

Visual depth:
Aim for visual depth in the texture. The surface of the texture should feel plastic and present in the game environment. With visual depth you can often compensate a low texture resolution. Some means for achieving this effect:
Painting in shades or highlights
Adding dirt, rust, scratches or other grunge layers.
Increasing contrast
Applying sharpen filter
Applying a bump - effect

Photos VS handpainted:
Avoid using photoresources in an unmodified shape. On the opposite, avoid an overly hand-painted look. Use your common-sense.

Models:

Armors and Clothing:
Aim for a vivid design. Clothing and armor often feature things like little pockets, belts, necklaces, collars, straps or other appendages. They are often more saturated and contrasted than other objects in the game environment. Fantasy designs that wouldn't work in the real world (huge pauldrons) are not a problem, but encouraged where they serve to characterize the culture this piece of armor/clothing is from. Cloth pieces usually reveal foldings that are more pronounced than the fabric structure (there are exceptions from this, mainly with robes). If possible, each piece of clothing and armor should be combinable with each other and be wearable by tall and wide races without revealing major issues.

Architecture:
Architecture is one of the main means to characterize an ethnical group, like the members of a great house or the inhabitants of a hold in skyrim. When designing architecture, think about attributes and the general character of this group and how they could be expressed over the architecture. Don't let yourself be constrained by realism-considerations too much there, only keep it in mind to avoid destroying other peoples suspension of disbelief. When creating architecture you should also think about how you want your architecture to be used in settlements, what kinds of layout would be possible with the structures you make. The layout should underline the general characteristic of your set. If the architecture will be found in a particular ecologic environment, consider including resources found in the latter (like the palm thatch roofs on gold coast houses of P:C).

Furniture and other objects of use:
Here apply similar considerations than in the architecture section. If the object you make, belongs to a certain ethnical group, use an according shape language and material.

Flora:
Plants, more than anything, should seamlessly fit into the environment they are placed in. They are preferably conceptualised together with other features of a planned environment and should follow its color pattern. What plants would be possible in this environment, where and how could they grow and what would they contribute?

Creatures:
No particular thoughts, except that I expect a creature to be at a certain minimum of quality, especially regarding its animation. Fantasy creatures and surreal designs are more valued than plain real world animals.

I agree with everything being said here. For creatures in specific, I would add that besides general quality and an appreciation for the surreal, we should also strive towards internally consistent ecosystems.

Some time ago Kaziem at TR created a catalogue with all kinds of vanilla textures as reference for modders to help us making textures that fit into the gameworld. This should be especially helpful to find the right level of saturation and contrast for own textures.